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An international team of astronomers reports the discovery of a new giant exoplanet around an M-dwarf star (GEMS). The newfound GEMS, designated TOI-5573 b, is comparable in size to Saturn and its mass is estimated to be 0.35 Jupiter masses. The findings were detailed in a research paper May 13 on the arXiv preprint server.

GEMS are a relatively rare population of extrasolar planets with radii about 8–15 times larger than Earth's and with masses of at least 80 times greater than that of our home planet. The formation of giant planets is challenging around M-dwarfs due to the smaller median masses of their protoplanetary disks. Given that these disks may not contain enough material to accrete the large cores necessary for giant planet formation, GEMS are generally rarely found.

TOI-5573 (also known as TIC 459762279) is an M dwarf around 40% smaller and less massive than the sun, located some 605.7 light years away. It was observed with NASA's Transiting Exoplanet Survey Satellite (TESS) several times, which resulted in the detection of a transit signal in its light curve. Now, a group of astronomers led by Rachel B. Fernandes of the Pennsylvania State University (PSU), has confirmed the planetary nature of this signal with follow-up observations.

"The planet was initially discovered by TESS and confirmed using a combination of 11 transits from four TESS Sectors (20, 21, 47, and 74), ground-based photometry from the Red Buttes Observatory, and high-precision radial velocity data from the Habitable-zone Planet Finder (HPF) and NEID spectrographs, achieving a 5σ precision on the planet's mass," the researchers wrote in the paper.

TOI-5573 b has a radius of approximately 0.87 Jupiter radii and its mass is about 0.35 Jupiter masses, which yields a density at a level of 0.66 g/cm³, therefore similar to that of Saturn. The planet orbits its host every 8.79 days at a distance of some 0.07 AU from it. The equilibrium temperature of TOI-5573 b was calculated to be 528 K.

The astronomers emphasized that TOI-5573 b is one of the coolest GEMS so far discovered, which makes it a valuable target for atmospheric characterization. They added that the super-solar metallicity of TOI-5573 suggests that metal-rich environments may play a significant role in shaping planet formation by increasing disk opacity and influencing gas accretion.

By comparing TOI-5573 b to other GEMS orbiting metal-rich stars, the authors of the paper concluded that giant planets preferentially form around M-dwarfs with super-solar metallicity. However, more observations are required in order to confirm this assumption.

"Further high-resolution spectroscopic analysis is needed to better understand how stellar metallicity affects the formation and properties of giant exoplanets like TOI-5573 b, as a more precise determination could provide deeper insights into the role metallicity plays in planet formation," the scientists noted.

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